The present invention relates to a process for heat-induced food coloring and a color system therefor. Using principles of food science, chemistry and physics, a system has been developed that may be used to color selected regions of foods upon exposure of a food to microwave radiation or heat by conventional cooking methods.
A major part of the appetizing appearance of conventionally heated foods is imparted by color, particularly brown, developed during preparation. Consumers have come to expect this brown appearance and consider it desirable in a variety of food products including meat, cheese, and cereal grain based products. It is not usually difficult to obtain a browned appearance under conventional oven cooking conditions because the reactions leading to brown colors will proceed in the presence of the components included in or added to the surface of most foods. However, it has been very difficult to obtain brown colors on the surfaces of foods prepared in microwave ovens without using browning devices.
Reasons why surfaces of microwave prepared products do not brown have been suggested (see for example, D.C.T. Pei, Baker's Digest, February 1982). This reference states that heat in a conventional oven is transmitted from the oven environment to the food surface via convection and transmitted from the surface to the interior of the product via conduction. This process of heat transfer enables the food surface to dehydrate and reach temperatures above the boiling point of water by the end of the conventional bake time. Microwaves, however, penetrate the surface and product and directly heat the interior of the product. This induces moisture transfer to the surface. Evaporation of the moisture from the surface to the microwave oven environment usually restricts the surface temperature to a maximum of about the boiling point of water during the microwave bake time. The resultant surface temperature is too low to enable the normal browning reactions to proceed at the necessary rate. In addition to the depressed rate of microwave browning versus conventional browning due to the temperature conditions, microwave preparation times are generally much shorter than conventional preparation times. Therefore, according to the aforementioned reference, the surface conditions and preparation times, resulting from the basic differences in heat transfer mechanisms between microwave and conventional heating, create a very difficult problem for those desiring to effect browning in a microwave oven.
Generally, the solutions to microwave browning can be divided into the following categories: packaging aided, cosmetic, and reactive coating approaches. The first approach involves the use of microwave susceptors which heat to temperatures exceeding the boiling point of water and brown surfaces in close proximity or direct contact (see for example, U.S. Pat. No. 4,266,108). Limitations of commercially available susceptors include the requirement of close proximity or direct contact, their generally uncontrolled temperature profile, and their generally high cost. The second approach is cosmetic and includes various surface applied formulations that are brown prior to application (U.S. Pat. No. 4,640,837, and U.S. patent application Ser. No. 055,851, Zimmerman et al.). The third approach involves coating the surface with a formula that will react to yield a brown color at the surface under microwave conditions described above. Two such variations of this approach are described in U.S. Pat. Nos. 4,735,812 and 4,448,791.
Advantages of the present invention over prior art include: control of color development prior to microwave heating thus allowing distribution of coated food products at any standard food distribution temperature (frozen, refrigerated, or shelf stable); use of the coating on many types of foods including those with dimensions that increase during manufacturing, distribution, or microwave heating (e.g., doughs and batters); the color system can be positionally stable therefore the end points of browning and textural development can be used to coincide; quantitative control of the color agents; dual microwave and conventional cooking applicability; tolerance to cooking time and conditions; and predictability.
The success of a product approach to browning requires control over the rate of the browning reaction. During the shelf life of a product, the rate of browning should be controlled or the product may brown prior to preparation by the consumer. Then, on exposure to a microwave field, the rate should be sufficiently high to brown the product during the short preparation times generally encountered with microwave products.
However, some of the above solutions to browning may result in various types of problems. The susceptor approach requires the addition of an additional packaging element or the use of an appliance in the microwave oven. The reactive dough layer disclosed in U.S. Pat. No. 4,448,791 by R. Fulde et al. requires the addition of a thin layer of a food product which poses some difficulties in handling. Further, the reactive dough layer system exhibits shelf life or distribution system related problems due to browning occurring prematurely and/or the color being diluted during freeze/thaw cycles. The last mentioned solution to browning, as all the other discussed references, are also limited only to the development or formation (production) of a brown color. The present invention is not so limited in that it can provide any desired color on the product when it is exposed to microwave radiation or heat while the color is substantially invisible to the consumer during shelf life and prior to preparation in the microwave oven. Further, the present invention can be utilized to provide color in any preselected region of the food, i.e. as a surface colorant and/or an interior colorant.
Other solutions to browning have been provided, for example, that set forth in the patent application entitled "Process for Microwave Browning" by D. Domingues et al. Ser. No. 07/339,567, filed Apr. 17, 1989, the disclosure of which is incorporated herein by reference. This latter system works very well with numerous starch-based items, particularly of the bread-type, for example, bread, biscuits, corn bread, quick breads, pastries, etc. It also provides the ability to reach a desired degree of brown coloration simultaneously with the desired texture development of the food substrate.